1. Field of the Invention
The present invention relates to technique of a color registration adjustment.
2. Description of the Related Art
There is a color image forming apparatus of an electrophotography system having image formation units that sequentially transfer images in different colors onto a transfer belt. A problem of the apparatus is that a temperature change inside the apparatus causes color misregistration because positions of overlapped images of the respective colors are not matched with each other due to distortions and deformations of lenses and mirrors.
In order to solve the above-mentioned problem, there is a known color registration adjustment that forms color registration patterns on a transfer belt at predetermined timing, detects amount of the color misregistration by reading the color registration patterns using a sensor, and controls image-writing-start timing etc. according to the detected amount of the color misregistration.
However, since the above-mentioned color registration adjustment needs to form the color registration patterns at suitable time intervals or for every predetermined number of printed sheets, down time increases. Against this problem, there is a suggestion that adjusts color registration without using a color registration pattern by holding correlation between internal temperature and an amount of color misregistration beforehand, and by predicting the amount of the color misregistration from the internal temperature. For example, United States Patent Application Publication 2008/0279599 discloses a technique that improves accuracy of the color registration adjustment without forming a color registration pattern by storing the amount of the color misregistration and the internal temperature that are detected by using the color registration pattern into a table in order to learn the color misregistration characteristic inherent in a color image forming apparatus.
The image forming apparatus disclosed in the above-mentioned publication holds the internal temperature when a generating condition of the color misregistration is satisfied and the corresponding amount of the color misregistration as a table.
However, since the generating condition of the color misregistration is generally satisfied when continuous operation time or the number of continuous printed sheets exceeds a predetermined value, the information on the table tends to incline toward the data in high internal temperatures. Such inclined data does not correctly reflect the color misregistration characteristic of the color image forming apparatus. There is a possibility to deteriorate the accuracy of the color registration adjustment at the internal temperatures with little information stored in the table.